NAME¶

std::make_heap - std::make_heap

Synopsis¶

Defined in header <algorithm>
template< class RandomIt > (1) (constexpr since C++20)
void make_heap( RandomIt first, RandomIt last );
template< class RandomIt, class Compare >
void make_heap( RandomIt first, RandomIt last, Compare (2) (constexpr since C++20)
comp );

Constructs a heap in the range [first, last).

1) The constructed heap is with respect to
operator<
(until C++20)
std::less{}
(since C++20).
2) The constructed heap is with respect to comp.

If any of the following conditions is satisfied, the behavior is undefined:

* The type of *first is not Swappable. (until C++11)
* RandomIt is not ValueSwappable.
* The type of *first is not MoveConstructible. (since C++11)
* The type of *first is not MoveAssignable.

Parameters¶

first, last - the range to make the heap from
comparison function object (i.e. an object that satisfies the
requirements of Compare) which returns true if the first argument is
less than the second.

The signature of the comparison function should be equivalent to the
following:

bool cmp(const Type1& a, const Type2& b);
comp -
While the signature does not need to have const&, the function must
not modify the objects passed to it and must be able to accept all
values of type (possibly const) Type1 and Type2 regardless of value
category (thus, Type1& is not allowed
, nor is Type1 unless for Type1 a move is equivalent to a copy
(since C++11)).
The types Type1 and Type2 must be such that an object of type
RandomIt can be dereferenced and then implicitly converted to both of
them.

Type requirements¶

-
RandomIt must meet the requirements of LegacyRandomAccessIterator.
-
Compare must meet the requirements of Compare.

Complexity¶

Given \(\scriptsize N\)N as std::distance(first, last):

1) At most \(\scriptsize 3N\)3N comparisons using
operator<
(until C++20)
std::less{}
(since C++20).
2) At most \(\scriptsize 3N\)3N applications of the comparison function comp.

Example¶

// Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <vector>

void print(std::string_view text, const std::vector<int>& v = {})
{
std::cout << text << ": ";
for (const auto& e : v)
std::cout << e << ' ';
std::cout << '\n';
}

int main()
{
print("Max heap");

std::vector<int> v{3, 2, 4, 1, 5, 9};
print("initially, v", v);

std::make_heap(v.begin(), v.end());
print("after make_heap, v", v);

std::pop_heap(v.begin(), v.end());
print("after pop_heap, v", v);

auto top = v.back();
v.pop_back();
print("former top element", {top});
print("after removing the former top element, v", v);

print("\nMin heap");

std::vector<int> v1{3, 2, 4, 1, 5, 9};
print("initially, v1", v1);

std::make_heap(v1.begin(), v1.end(), std::greater<>{});
print("after make_heap, v1", v1);

std::pop_heap(v1.begin(), v1.end(), std::greater<>{});
print("after pop_heap, v1", v1);

auto top1 = v1.back();
v1.pop_back();
print("former top element", {top1});
print("after removing the former top element, v1", v1);
}

Output:¶

Max heap:
initially, v: 3 2 4 1 5 9
after make_heap, v: 9 5 4 1 2 3
after pop_heap, v: 5 3 4 1 2 9
former top element: 9
after removing the former top element, v: 5 3 4 1 2

Min heap:
initially, v1: 3 2 4 1 5 9
after make_heap, v1: 1 2 4 3 5 9
after pop_heap, v1: 2 3 4 9 5 1
former top element: 1
after removing the former top element, v1: 2 3 4 9 5

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3032 C++98 the elements of [first, last) was not required required
to be swappable

See also¶

is_heap checks if the given range is a max heap
(C++11) (function template)
is_heap_until finds the largest subrange that is a max heap
(C++11) (function template)
push_heap adds an element to a max heap
(function template)
pop_heap removes the largest element from a max heap
(function template)
turns a max heap into a range of elements sorted in ascending
sort_heap order
(function template)
priority_queue adapts a container to provide priority queue
(class template)
ranges::make_heap creates a max heap out of a range of elements
(C++20) (niebloid)